1. Field of the Invention
This invention relates to a magnetic material, more particularly to a magnetic material that has a nanometer-scale pinning effect.
2. Description of the Related Art
A magnet ic material that is applied to a perpendicular magnetic recording medium, a patterned medium, a spintronic device, etc., preferably has an ordered crystal structure and superior perpendicular magnetocrystalline anisotropy (Ku).
The applicants of this invention published an article concerning the entire evolution of the structure and magnetic properties of a sputter-deposited CoPt thin film on a MgO(111) substrate (Scripta Materialia, 61 (2009), 713-716). The CoPt thin film was deposited on the MgO (111) substrate using dc magnetron sputtering, followed by annealing the CoPt thin film from room temperature (RT) to 750° C. so as to examine the phase transformation of the CoPt thin film within the temperature range. Analyzed by X-ray diffractometry (XRD), the CoPt thin film had a crystal structure of disordered A1 phase when the annealing temperature (Ts) is greater than 25° C. and less than 250° C. When 250° C.<Ts<350° C., the crystal structure was transformed into L11 phase that has rhombohedral lattices (lattice parameter: a=b=c and α=β=γ≠90°. When 350° C.<Ts<600° C., the crystal structure was replaced by disordered A1 phase. As Ts became higher than 600° C., the crystal structure was transformed into L10 phase of face-centered tetragonal (fct) AuCu-type structure (lattice parameter: a=b≠c and α=β=γ=90°.
Referring to FIG. 1, the rhombohedral lattice of the L11 phase of the CoPt thin film in the temperature range of 250° C.-350° C. has a unit cell consisting of alternating close-packed atomic layers of Co and Pt along the [111] direction, thereby resulting in a magnetic easy-axis parallel to the [111] direction. The L11 phase has a relatively low out-of-plane coercivity (Hc⊥) of 39.8 kA/m, i.e., about 0.5 kOe, which is deficient for a patterned medium that generally requires an out-of-plane coercivity of 0.8-5 kOe and a saturation magnetization (Ms) of 400-900 emu/cm3.
Ji Hyun Min et al. disclosed Cu-doped CoPt nanowires having a composition of Co58Cu16Pt23 and a face centered cubic (fcc)-Co3Pt phase (lattice parameter: a=b=c and α=β=γ=90° prior to annealing analyzed by energy dispersive spectroscopy (EDS) (see Journal of Applied Physics, 101, 09K513, 2007). After annealing at 500° C., minor hexagonal close-packed (hcp)-CoPt phase (lattice parameter: α=β≠c and α=β=90°≠γ) appeared in the Cu-doped CoPt nanowires, with the fcc-Co3Pt phase retained therein. According to the analysis of hysteresis loops, the Cu-doped CoPt nanowires had an out-of-plane coercivity (Hc⊥) of about 850 Oe, which may conform to the requirements of a patterned medium and a spintronic device (Hc⊥<2 kOe). However, the XRD analysis showed that the annealed Cu-doped CoPt nanowires in this article had no preferred orientation parallel to the nanowire axis (the easy axis), thereby resulting in the inapplicability of the Cu-doped CoPt nanowires in a spintronic device.
Therefore, it is desired in the art to provide a CoPt alloy that has a superior out-of-plane coercivity (Hc⊥) and a preferred orientation parallel to the easy axis of the alloy such that the CoPt alloy may be applied to a patterned medium and a spintronic device.